Drumming In The Message.

Adding yellow to the highlights accentuates the glare of the sun. Increasing the density of all of the colors in the midtones and shadows adds to the contrast of the image and makes the sunlight even stronger.

Using Web Graphics Default in Color Settings, the grayscale image was converted to CMYK. One layer was filled with C30 M45 and Y100 and assigned a Blend Mode of Color. After selecting the original grayscale, another layer was filled with the same color and assigned a Blend Mode of Multiply and Opacity of 25% to add extra weight to the highlights and midtones. Using Color Range, a shadow was selected with the eyedropper.

More info:

Quadtones »
Tonal Range »

Ed #2 Quadtones

Image Credit: Andrew Martin